The proposed investigations include three related areas: 1) the interaction of steroid hormone-receptor complexes with nucleotides; 2) influence of nucleotides in the process of transformation of cytosol receptors to a nuclear binding form; 3) the interaction of activated hormone-receptor complexes with nuclear components of the target tissues. Steroid rceptors have been shown to interact with ATP. This interaction has been studied in crude cytosol preparations from target cells for progesterone, estrogen and glucocorticoids. Efforts will be made to study this nucleotide-receptor interaction in purified progesterone receptor preparations. ATP presence has been shown to promote in vitro receptor activation at 0-4 C in salt-free conditions. The activated receptor shows increased affinity to bind to DNA-cellulose, isolated nuclei, and ATP-Sepharose. Studies will be performed to determine whether or not similar binding sites on the activated receptor are involved in their uptake by these receptors; and to investigate if the nucleotide-dependent activation of steroid-receptors involve physio-chemical changes similar to those brought about by conventional procedures like heat- and salt-activation. In recent years certain phosphatase inhibitors (molybdate, tungstate, vanadate) have been identified which stabilize the steroid-receptors but selectively block the process or receptor activation. These studies have advanced the hypothesis that phosphorylation and dephosphorylation of the receptor or a regulatory molecule may be involved in the process of steroid binding and receptor activation, respectively. Studies will be designed to test this hypothesis and to determine whether these phosphate inhibitors act directly by interacting with the receptor molecules. These studies are expected to provide correlation between such events as hormones binding, activation of steroid-receptor complex, and the interaction of the activated receptor with nuclear sites. Such information will enhance our understanding of the molecular aspects of the process of reproduction.